The present invention relates to a method and a device for the detection of a pressure drop in a tire of a wheel of a vehicle.
It is desirable for many reasons to know about the pressure in the individual wheels. On the one hand, it is per se unsafe to drive with insufficient tire pressure so that an information to the driver about insufficient tire pressure on any one or a specific wheel is desirable. In addition, modern driving dynamics control systems are based on defined force transmission mechanisms between roadway and tires. The operating points of the control strategies are rated in conformity with these assumptions. However, in case the tire pressure in a wheel is not correct and especially too low, the preconditions which are the basis of driving dynamics control operations no longer apply, and the said operations are either poorly adapted to the actual situation, or may even be partly detrimental. This means that having knowledge about tire pressure conditions is also desirable to influence control strategies of driving dynamics control in modern automotive vehicles.
Tire pressures may be gathered directly by sensors. However, this necessitates additional effort and structure in sensors and cables. In addition, a serial interface must be provided between the rotating wheel and the stationary, other hardware. This is complicated and frequently also sensitive to disturbances. Another method is directed to deriving tire pressure conditions from the wheel signals. Wheel signals are wheel-related sensor signals which contain an information about the wheel angle speed. These wheel signals are already provided in up-to-date vehicle dynamics control systems because the said signals are required for functions such as ABS or ESP (Electronic Stability Program). The wheel signals may be a pulse sequence having a frequency which is a standard of the rotational speed of the wheel.
The mechanism by which tire pressure conditions may be inferred from the wheel signals is explained by way of FIG. 1. Reference numeral 11 refers to a wheel on a roadway 13. The vehicle moves at a vehicle speed Vf. When in its proper condition, the wheel rotates at the angular speed Wo. Vf and Wo correlate on the basis of the formula Wo=Vf/Ro, wherein xcex94Ro is the dynamic rolling radius xcex94Rdyn with a normal tire pressure of wheel 11. Reference numeral 12 refers to a wheel with pressure loss. Because the tire pressure is lower, the wheel load, however, remains equal in a first approximation, the tire contact becomes longer, more tire profile is pressed on the roadway 13 so that the dynamic rolling radius becomes smaller in the error case Rf. However, because the faulty wheel is moved along at the same vehicle speed Vf as the proper wheel, an angular speed Wf will be achieved for the defective wheel corresponding to the above-mentioned formula being in excess of the angular speed Wo of the proper wheel. Because the wheel angle speed is reflected in the wheel signals, an information with respect to the tire pressure may thus be gathered from the wheel signals. However, precautions must be taken to avoid wrong detections.
It is a strategy known in the art to determine a test quantity with respect to the wheel radii of all wheels of the vehicle, and to check it then. The determination is effected so that an average of the disturbance variables may be formed, if necessary. Instead of the wheel radii, equivalent values may be used, for example, the wheel angle speeds (inversely proportional to the respective wheel radii) or the reverse value of the wheel angle speeds (directly proportional to the wheel radii).
Cornering maneuvers are e.g. considered as disturbance variables. The wheels on the outward side of the bend have to cover a greater radius than those on the inward side of the bend so that the wheels on the outward side of the bend will exhibit a higher frequency of their wheel signals, without this fact being due to a pressure drop in a tire. Traction slip is another disturbance variable. The wheels on the driven axle will have a low slip so that also these wheels have a higher circumferential speed compared to the non-driven wheels, without this fact being due to a tire pressure loss on the wheels of the driven axle.
One test variable which, in a certain way, is insusceptible to such disturbances may e.g. be determined as follows:   PG  =            W1      +      W3              W2      +      W4      
wherein W1 is the angular speed of the wheel 14 in the vehicle 10 of FIG. 2, W2 is the angular speed of the wheel 15, W3 is the angular speed of the wheel 16, and W4 is the angular speed of the wheel 17. A diagonal pair combination of the wheels, summation within the pairs, and calculation of quotients of the sums is effected. This way, at least some systematic faults will be averaged which are not due to tire pressure drops. The test variable may then be checked in its value. Conclusions with respect to tire pressure conditions may be drawn therefrom. In the ideal case (all wheels are of equal size and have a correct pressure), the quotient is 1. When one wheel loses pressure, its radius or angular speed will vary, and accordingly also the quotient will differ from the ideal value. Details concerning tire pressure conditions may be inferred from the direction of the discrepancy. When e.g. in the above-mentioned formula the test quantity is different from the normal value 1 and now adopts value 1.1, this can mean that either the angular speed of one of the wheels 1 or 3 has increased or that the angular speed of one of the wheels 2 or 4 has decreased. The latter may also occur, for example, when the sun shines on a tire, the said is heated thereby, its inner pressure and, hence, its radius rises and its angular speed decreases.
However, the above-mentioned method also is susceptible to disturbances in some respects.
An object of the present invention is to provide a method and device for the detection of a pressure drop in a tire of a wheel having a reliable operation.
This object is achieved by a method of detection of a pressure drop in a tire of a wheel, comprising the following steps:
determining a test variable from the wheel radii or from variables representing these wheel radii of the wheels of the vehicle, wherein the test variable is a quotient of two sums of each two wheel radii or variables representing these wheel radii of each one pair of vehicle wheels,
comparing the test variable with a threshold value, and
detection of a pressure drop with respect to the result of the comparison,
several test variables are determined with respect to several different wheel pair combinations, and
pressure drop is considered to have been identified only if a defined vehicle wheel is identified by all found test variables as a wheel with a possible pressure drop.
According to the present invention, several test variables are determined with respect to several different wheel pair combinations, and a pressure drop is considered to have been identified only if a defined vehicle wheel is identified by all found test variables as a wheel with a possible pressure drop.
It is possible to employ the method insofar in a two-stage way as initially only one test variable is determined, and only if this test variable indicates a possible pressure drop will further test variables be determined and evaluated.
The different pair combinations may be: corresponding to the vehicle diagonals, corresponding to the vehicle axles, corresponding to the vehicle sides.
The test variable(s) can be determined with respect to possibly learnt correction values. The correction values can be dependent on driving dynamics so that also the correction of the test variable is carried out in dependence on driving dynamics.
When comparing the test variable, time considerations may also be taken into account, for example, to such effect as to whether the test quantity has reached or exceeded the threshold value for a defined time portion within a defined period of time, or whether the test variable has reached or exceeded the threshold value beyond a defined period of time. Only if these time conditions are also satisfied will it be considered as an identification that a threshold value has been reached or exceeded, while single events will not yet cause this identification.